Conventional high-temperature bipolar batteries have positive and negative electrodes which are confined relative to the collectors of positive and negative current. The current collectors are electrically insulated from one another by a separator. An electrolyte is present and infused throughout (into) the electrodes and the separator. The positive and negative current collectors are commonly formed of electrically conductive sheets which also confine the electrode materials. When arranged in a series configuration, a bipolar plate caps the negative electrode, as well as attaches the positive compartment of the next cell in the bipolar stack. Full size batteries of this type are comprised of many cells grouped together in an end-to-end or face-to-face arrangement in a common battery housing and are electrically connected in series to produce higher effective voltage output. Typically, the negative electrode material is a lithium alloy, such as LiSi or LiAl and the positive electrode material is an iron sulfide, such as FeS or FeS.sub.2.
Electrolytes of the prior art include mixtures of lithium chloride and potassium chloride. Such systems exhibit a relatively limited dynamic range--the ratio of alkali or alkaline earth metal ions over which the electrolyte will remain liquid--at a specific temperature. A limited dynamic range manifests itself in electrolyte solidification in the electrodes as the concentration of positive ions changes during cell operation. As a result, the larger the dynamic range, the more useful the electrolyte, inasmuch as solidification reduces electrode efficiency.
The search for an effective, efficient electrolyte system has been an on-going concern of the prior art. One approach, which has been used with some success, involves the use of a eutectic mixture of lithium chloride, lithium bromide, and potassium bromide, as disclosed in U.S. Pat. No. 4,764,437 issued Aug. 16, 1988, the entire disclosure of which is incorporated herein by reference. Therein a low melting electrolyte having an expanded dynamic range is described as part of a cell demonstrating overall improved discharge capacity. It was found that using the eutectic in combination with a dense FeS.sub.2 electrode provides lower effective operating temperatures and eliminates the capacity-loss problem typically associated with lithium alloy cells using a lithium chloride/potassium chloride electrolyte. With lower operating temperatures and an expanded electrolyte liquidus range, electrolyte solidification in the electrodes is reduced at high current densities.
However, the prior art is associated with several problems, most of which are related to the separator apparatus used in conjunction with the electrolytic material. Conventional separators are formed from fibrous boron nitride or, alteratively, pressed magnesium oxide or aluminum nitride powder. For example, an electrolyte/separator of the prior art is typically fabricated by introducing molten electrolyte into porous magnesium oxide, followed by a high-pressure, cold-pressing operation. The deficiencies of such separators are well known: the separators are necessarily restricted to relatively-thick designs and configurations, high-pressure fabrication is relatively costly and introduces mechanical stress upon ejection, and capacity utilization is reduced at higher current densities.
Accordingly, it is an object of the invention to provide an improved separator for bipolar batteries.
It is an object of the invention to provide a separator which is effectively distributed in the electrolyte, by melting both the electrolyte phase and powder-separator phase, to form a molten two-phase mixture at battery operating temperatures.
It is another object of this invention to provide a quaternary salt system, in which a binary salt is generally immiscible with a eutectic mixture of three electrolytic binary salts.
Another object of this invention is to provide a separator for use in conjunction with a eutectic electrolytic mixture for improved performance at higher current densities, as compared to separators and related apparatus of the prior art.
It is another object of this invention to provide an alternative electrolyte/separator fabrication, one involving melting both electrolyte and separator phases for casting into molds, such that separators may be designed and configured for a variety of electrochemical applications.
A further object of this invention is to provide a binary separator salt having low solubility in a eutectic electrolytic mixture, such that battery operating temperatures will sinter the salt in sire, physically strengthening the separator through operation.
Another object of this invention is to provide an electrolyte/separator such that a cell may exhibit improved performance, greater capacity, and maximum utilization, even at low current densities.
Another object of this invention is to provide a new electrolyte/separator such that batteries may be designed with thicker electrodes, reducing cost and increasing specific energy.
Another object of this invention is to increase the fraction of molten electrolyte in the separator to increase ionic conductivity.
Another object of this invention is to increase lithium ion activity in the electrolyte to enhance electrode performance, but maintain low battery operating temperatures and electrolyte liquidus.
Another object of this invention is to maintain a stable Li.sup.+ /K.sup.+ ratio at high current densities to enhance cell performance.
These and other important objects of the invention will be apparent from the following description.